The implementation discussed here sounds like it runs directly on the metal, or at least as close to the metal as IBM lets you get on those machines. IBM's main

The implementation discussed here sounds like it runs directly on the metal, or at least as close to the metal as IBM lets you get on those machines. IBM's mainframe OS (MVS then, z/OS now) also includes a different a UNIX implementation called USS (UNIX System Services) - this works in a similar way to the way WSL 1 did, and acts as an emulation layer that allows you to compile and run UNIX software on top of the mainframe OS, and do things like using a mainframe dataset as a mounted filesystem. It even includes an interactive shell implementation that runs on a 3270 terminal. I briefly worked for a large mainframe software vendor on a port of OpenLDAP that used USS. There's some real funky things going on in the emulation layer; bytes received from TCP sockets are automatically re-encoded from ASCII to EBCDIC before your program sees them, and bytes you send are automatically re-encoded from EBCDIC to ASCII. That made working on network code lots of "fun" since there are a lot of folks that seem to like to hardcode the values for ASCII CR and LF instead of using "\r" and "\n".